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AMAZON multi-meters discounts AMAZON oscilloscope discounts Objectives: • Explain magnetism and the part it plays in the operation of electric motors. • Explain torque and the purpose of different types of single-phase motors. • Explain the operation of a basic electric motor. • Understand how to operate, install, reverse the rotation (if possible), and diagnose problems in a shaded-pole motor. • Understand the purpose of capacitors in the operation of a single-phase motor and be able to explain the difference between a starting and running capacitor. • Correctly diagnose the condition of any capacitor and , using capacitor rules, be able to substitute a capacitor if a direct replacement isn't available. • Explain how to operate, install, troubleshoot, and repair (if possible) split-phase and capacitor-start motors. • Explain how to operate, install, troubleshoot, and repair (if possible) permanent split-capacitor motors. • Explain how to operate, install, troubleshoot, and repair (if possible) capacitor-start—capacitor-run motors. • Understand how to operate, install, reverse, and troubleshoot three-phase motors. • Explain how to operate, install, troubleshoot, and repair (if possible) electronically commutated motors. • Identify the common, start, and run terminals of a single-phase compressor motor. KEY TERMS:
INTRODUCTION The electric motor changes electric energy into mechanical energy. Motors are used to drive compressors, fans, pumps, dampers, and any other device that needs energy to power its movement. There are many different types of electric motors with different running and starting characteristics. Most single-phase motors are designed and used according to their running and starting torque. Torque is the strength that a motor produces by turning, either while starting or running. This section covers most types of motors available today and how they are used in the heating, cooling, and refrigeration industry. All electric motors should be properly grounded. We begin our study with a discussion of magnetism, an effect that's needed to operate motors, relays, contactors, and other electric devices. MAGNETISM Magnetism is the physical phenomenon that includes the attraction of an object for iron and is exhibited by a permanent magnet or an electric cur rent. Magnetism is produced in many different ways, but regardless of how it's produced, the effect is basically the same. The magnetic field of the earth, for example, is the same as the magnetism in a horseshoe magnet, the magnetism produced by a transformer, and the magnetism produced by an electromagnet. A good example of magnetism is the ability of a horse shoe magnet to pick up articles made of iron. The most common example of magnetism is the reaction of a compass to the earth’s magnetic field. All magnets have two poles, a north pole and a south pole. If the north pole of a bar magnet is brought close to the north pole of another bar magnet, the magnets will repel each other, as shown in ill 9.1. If the south pole of one bar magnet is brought close to the north pole of another bar magnet, the magnets will attract each other and come together, as shown in ill 9.2. Therefore, like poles of magnets repel each other and unlike poles attract. ill. 9.1 Repulsion of like poles of two bar magnets ; ill. 9.2 Attraction of unlike poles of two bar magnets Magnetic Field The magnetic lines of force of a magnet that flow between the north and south poles are called flux. These lines of force are highlighted in ill 9.3. The area that the magnetic force operates in is called a magnetic field. Magnetic fields can flow through materials, depending on the strength of the magnetic field. A magnetic field is best conducted through soft iron. That is why certain parts of motors and other electric devices are made of soft iron. Induced Magnetism Induced magnetism is created when a piece of iron is placed in a magnetic field. The important fact to remember about a magnetic field is that the closer an object is to the magnet, the stronger the magnetic field is on that object. Therefore, if we insert an iron bar within 2 or 3 inches of a magnetic field, we induce a stronger field than if we placed the bar 6 inches from the field. Two types of magnets are in use today: the permanent magnet and the electromagnet. The permanent magnet is a piece of magnetic material that has been magnetized and can hold its magnetic strength for a reasonable length of time. The permanent magnet must be made of a magnetic material, such as iron, nickel, cobalt, or chromium. Some nonmagnetic materials, such as glass, rock, wood, paper, and air, cannot be made magnetic but can be penetrated by a magnetic field. ill. 9.3 Magnetic field of a bar magnet; ill. 9.4 Magnetic field created around a current-carrying conductor The electromagnet is a magnet produced through electricity. When an electron flow is in a conductor, a magnetic field is created around the conductor, as highlighted in ill 9.4. The larger the electron flow, the stronger the magnetic field. Therefore, if we take an iron core and wind a current-carrying conductor around it, the iron core will become a magnet, as shown in ill 9.5. In ill 9.5 the magnetic field is highlighted. The electron flow and the number of turns of the conductor around the core determine the strength of an electromagnet. ill 9.6 shows an electro magnet that's used as a solenoid in a contactor. Magnetism is important in the heating, cooling, and refrigeration industry because of its many uses in the operation of electric devices. Motors require magnetism to create a rotating motion. Relays and contactors use magnetism to open and close a set of contacts. All of the devices discussed in this section use magnetism in some way. ill. 9.5 Magnetic field of an iron core when a current-carrying conductor is wound around the core ill. 9.6 An electromagnet used as a solenoid in a contactor BASIC ELECTRIC MOTORS Electric motors are common devices in the heating, cooling, and refrigeration industry. Motors are used to create a rotating motion and drive components that need to be turned. Motors power compressors, pumps, fans, timers, and any other device that must be driven with a rotating motion. In an electric motor, electric energy is changed to mechanical energy by magnetism, which causes the motor to turn. The method by which magnetism causes motors to rotate uses the principle that like poles of magnets repel and unlike poles attract. Suppose a simple magnet is placed on a pivot and used as a rotor (the rotating part of an electric motor) and a horseshoe magnet is used as a stator (the stationary part of a motor), as shown in ill 9.7(a). Movement will be obtained by the repulsion and attraction of the poles of a magnet. The rotor would turn until the unlike poles are attracted to each other, as shown in ill 9.7(b). ill. 9.7 A simple electric motor: (a) Initial position of the rotor; (b) Movement of the rotor due to the repulsion and attraction of the magnets poles ill. 9.8 Complete cycle of operation of an electric motor: Two-pole motor, 3600 rpm. Beginning of cycle, One-half of cycle, Full cycle To make an electric motor move continuously, we must have a rotating magnetic field, which is produced by the reversal of the poles, or the polarity, in the rotor or stator. An alternating current of 60 hertz changes direction 120 times per second. Therefore, the current would change the polarity of the stator poles on each reversal of current. If the rotor has a permanent polarity, as shown in ill 9.8, then the changes of polarity in the stator would cause the rotor to move. Therefore, if alternating current changes direction, causing a polarity change, 120 times a second, then the motor will turn in a continuous motion because the poles of the stator will be continuously repelling and attracting the permanent poles of the rotor. ill 9.8 shows the motor in one complete cycle of current or one-sixtieth of a second. The movement of the motor is caused by the magnetic field of the stator as it rotates through its alternations of current. ill 9.9 shows the windings of an ordinary electric motor. The windings of the motor are a part of the stator. ill. 9.9 Windings of an electric motor; ill. 9.10 Squirrel cage rotor In motors, the rotor isn't a permanent magnet, as we stated in the previous explanation. The squirrel cage rotor shown in ill 9.10 is the most commonly used rotor today. The squirrel cage rotor derives its name from its cagelike appearance. In the squirrel cage rotor, copper or aluminum bars are evenly spaced in the steel portion of the rotor and connected by an aluminum or copper end ring. The squirrel cage rotor produces an inductive magnetic field within itself when the stator is energized. The most common motors operate much like a transformer, with the stator being the primary magnetic field and the rotor being a movable secondary magnetic field. The rotor will have magnetism induced into it from the stator, and its magnetic poles will be permanent. The magnetic poles of the stator are moving at the rate of the alternations of the current. TYPES OF ELECTRIC MOTORS The industry uses all kinds of AC motors to rotate the many different devices that require rotation in a complete system. Different motors are needed for different tasks because not all motors have the same running and starting characteristics. This fact, along with the increased cost of stronger motors, allows the industry to use the right motor for the right job. E.g., many compressors require a motor with a high starting torque and a good running efficiency. Small propeller fans use motors with a low starting torque and average running efficiency. Motor Strength The starting methods, or strengths, are generally used to classify motors into types. Motors are selected mainly because of the starting torque (power) required for the motor to perform its function. Six general types of motors are used in environmental systems: shaded pole, split phase, permanent split capacitor, capacitor start—capacitor run and capacitor start, three phase, and electronically commutated. There are others, such as the repulsion- start—induction-run and series motors. However, these are outdated or not commonly used in the industry. The starting torques of the six general types of induction motors, expressed as a percentage of their running torque, are as follows: shaded pole, 100%; split phase, 200%; permanent split capacitor, 200%; capacitor start—capacitor run and capacitor start, 300%; three phase, 600%; and electronically commutated, 200%. Motor Speed The following formula can be used to determine the speed of an electric motor with a load: Speed = (Flow reversals/second(hertz) x 120) / (Number of poles) One cycle of alternating current has two flow reversals. If 60 hertz alternating current is being used, there are 120 flow reversals per second. E.g., if a four-pole motor is used in an application, its calculated rpm is: Four-pole motor speed = (60 x 120) / 4 = 1800 The actual rpm of a four-pole motor is 1750 rpm. Motor speeds that are common to the industry are Two-pole motors: 3450 rpm Four-pole motors: 1750 rpm Six-pole motors: 1050 rpm Eight-pole motors: 900 rpm Open and Enclosed Motors Motors are commonly either open or enclosed. The open motor, shown in ill 9.11, has a housing and is used to rotate a device such as a fan or a pump that's itself not enclosed in any type of housing. The enclosed motor is housed within some type of shell. The most common enclosure of a motor is a completely sealed hermetic compressor, as shown in ill 9.12. Any starting apparatus used on an enclosed motor must be mounted outside the enclosure. The starting apparatus of an open motor is usually mounted within the motor itself. ill. 9.11 Open motor; ill. 9.12 Enclosed motor used in a hermetic compressor In the following sections, we will discuss the six basic motor types in detail. Motor Dimensions The National Electrical Manufacturers’ Association (NEMA) has established standard motor dimensions. The standards are useful when a technician is forced to locate a replacement motor for a particular application. SHADED-POLE MOTORS Most single-phase induction motors require a starting winding to create a starting torque that enables the motor to start. In most cases, the starting winding is located 90 electrical degrees from the main winding. A shaded- pole motor uses a shaded pole made of a closed turn of a heavy copper wire banded around a section of each stator pole. A shaded-pole motor is shown in ill 9.13. Shaded-pole motors are used when very small starting and running torques are required, such as in a furnace fan, a small condensing unit fan, and an open-type propeller fan. These motors are easily stalled, but in most cases, because of the small locked rotor amperes (i.e., the current draw of the motor when power is applied but the motor does not turn), they can stall and still not burn out the windings. ill. 9.13 Shaded-pole motor ill. 9.14 Stator and rotor of shaded-pole motor; ill. 9.15 Shaded pole Operation ill 9.14 shows the stator of a shaded-pole motor. At one side of each pole, a small groove has been cut into the stator and banded by a solid cop per wire or band, as shown in ill 9.15. When the shaded-pole motor is starting, a current is induced into the shaded pole from the main windings. The shaded poles produce a magnetic field that's out of phase with the magnetic field of the main winding, and a rotating magnetic field is produced that's sufficient to give the desired starting torque. When the motor approaches full speed, the effect of the shaded pole is negligible. The rotation of the shaded-pole motor is from the unshaded edge of the pole toward the shaded edge of the pole. ill 9.16 shows the schematic diagram of a single-speed, shaded- pole motor. A single-speed, shaded-pole motor has only one winding, with the exception of the shaded pole, and is relatively simple. ill 9.17 shows the schematic diagram of a three-speed, shaded-pole motor. The main winding is the speed winding for high-speed operation. For medium-speed operation, the main winding is put in series with the medium-speed winding, which increases the number of poles and produces fewer revolutions per minute. For low-speed operation, the main winding is put in series with the medium- and low-speed windings, which increases the number of poles in the stator and further reduces speed. ill. 9.16 Schematic wiring diagram of the shaded-pole motor; ill. 9.17 Schematic diagram of three-speed, shaded-pole motor Reversing Shaded-pole motors are difficult to reverse because to do so, you must disassemble them. The rotation of the shaded-pole motor is determined by the location of the shaded poles. ill 9.18 shows a layout of a single-speed, shaded-pole motor. When the shaded poles are on the left side of the main poles, as in ill 9.18, the rotation will be toward the shaded poles, or clockwise. On the other hand, when the shaded poles are on the right side of the main poles, as shown in ill 9.19, the rotation will again be toward the shaded poles, but in this case, the rotation will be counterclockwise. Therefore, for reversing the shaded-pole motor, the stator must be reversed to change the positions of the shaded poles, and this usually means disassembling the motor. ill. 9.18 Layout of a shaded-pole motor, with clockwise rotation in the direction of the shaded poles: Rotor, Main poles ill. 9.19 Layout of a shaded-pole motor, with counterclockwise rotation in the direction of the shaded poles Troubleshooting Shaded-pole motors are easy to identify because of the copper band around the shaded pole, previously shown in ill 9.15. A single-speed, shaded- pole motor is easily diagnosed for trouble because of its simple winding patterns, previously shown schematically in ill 9.16. Multispeed shaded-pole motors are more difficult to troubleshoot because of the additional speed windings, which were previously shown in ill 9.17. The shaded-pole motor can be checked with an ohmmeter to determine the condition of the windings. Because a shaded-pole motor has stalled does not mean the windings are faulty. If this condition should occur, the motor probably needs lubrication. The shaded-pole motor is simple and easy to troubleshoot. It is used in many applications in the industry. ill. 9.20 Symbol for a capacitor; ill. 9.21 Common capacitors used in the industry: (a) Starting capacitors; (b) Running capacitors CAPACITORS The capacitor consists of two aluminum plates with an insulator between them. The insulator prevents electrons from flowing from one plate to the other, but it permits the storage of electrons. ill 9.20 shows the schematic symbol for a capacitor. Capacitors are used to boost the starting torque or running efficiency of single-phase motors. Two Types Used in the Industry Two types of capacitors are used primarily in the industry: the electrolytic or starting capacitor and oil-filled or running capacitor (ill 9.21). Starting capacitors are usually in a plastic case consisting of two aluminum electrodes (plates) with a chemically treated paper, impregnated with a nonconductive electrolyte, between them. They can be purchased in ranges from 75 to 600 microfarads (uF) and from 120 to 300 volts. A microfarad is the unit of measurement for the strength of a capacitor; all capacitors are rated according to their strength in microfarads. The electrolytic capacitor is used to assist a single-phase motor in starting. The oil-filled capacitor consists of two aluminum electrodes with paper between them and an oil-filled capacitor case. It is available in microfarad ranges of about 2 to 60 and voltage ranges of 240 to 550. The oil-filled capacitor can be used for small or moderate torque starting, but it's more commonly used to increase a motor’s running efficiency. The major difference between the two types of capacitors is in their application. A starting capacitor is built in a relatively small case with a dielectric—a nonconductor of electric current. It is used for only a short period of time on each cycle of the motor. Therefore, a starting capacitor has no need to dissipate heat, although its capacity is larger than that of its counterpart, the running capacitor. The running capacitor is designed to stay in the motor circuit for the entire cycle of operation. Therefore, it must have some means of dissipating the heat. The oil in the capacitor case is used for this purpose. The oil-filled capacitor is physically larger than the starting capacitor but smaller in capacity than the starting capacitor and usually contained in a metal case. Both capacitors are in wide use in the industry. Troubleshooting Short capacitor life and malfunctions can be caused by several different factors. High voltage can cause a capacitor to overheat. This can damage the plates and short the electrodes. Starting capacitors can be damaged by faulty starting apparatus that would keep the capacitor in the line circuit long enough to damage the capacitor. Excessive temperature can shorten the life of capacitors or cause permanent damage due to poor ventilation, starting cycles that are too long, or starting cycles that occur too frequently. The cause of the malfunction should be corrected as soon as possible. The capacitors themselves are frequently the cause of the problem. CAUTION: Before handling or checking a motor capacitor, short from one terminal to another with a 20,000-ohm, 4-watt resistor. All capacitors used on single-phase motors are designed specifically to assist the motor in proper operation. However, in some cases it's impossible to replace a capacitor with an exact replacement. If this situation should occur, use the following guidelines for replacing the capacitor: 1. The voltage of any capacitor used for replacement must be equal to or greater than that of the capacitor being replaced. 2. The strength of the starting capacitor replacement must be at least equal to but not more than 20% greater than that of the capacitor being replaced. 3. The strength of the running capacitor replacement may vary by plus or minus 10% of the strength of the capacitor being replaced. 4. If capacitors are installed in parallel, the sum of the capacitors is the total capacitance. 5. The total capacitance of capacitors in series may be found in the following formula: C=(C1 x C2) / (C1 + C2) These rules are intended only as a guide. Remember, it's always prefer able to use an exact replacement. CAUTION: When making electrical connections to a running capacitor, make sure that power supplying the capacitor is connected to the marked terminal. Many methods for testing capacitors are in common use in the industry today. A capacitor can be checked by using an ohmmeter. The ohmmeter should be placed on a high-ohm scale and both leads should be connected to the terminals of a discharged capacitor. If the needle of the meter shows a deflection to the right end of the scale and back to infinity, the capacitor is probably good. If the needle comes to rest on 0 ohms, the capacitor is shorted. If the needle of the meter does not move, the capacitor is open. In case of doubt, another method can be used to check the capacitor. By briefly applying voltage to a capacitor, reading the amperage, and then substituting the values into the following formula, we can obtain the exact capacitance: microfarads = (2650 x amperes) / volts When performing this test, put a fuse in the circuit to prevent overloading due to a shorted capacitor, as shown in ill 9.22. Starting capacitors should be put in the circuit for approximately five seconds only. Many commercial capacitance testers are available on the market, one of which is shown in ill 9.23. Many new digital volt-ohm meters test capacitors. ill 9.24 shows a digital volt-ohm meter that can check the capacitance of a capacitor. ill. 9.22 Schematic diagram of electric circuit to check the capacitance of a capacitor ill. 9.23 Capacitor tester; ill. 9.24 Digital volt-ohm meter capable of reading the capacitance of a capacitor . |